This invention relates to a method for fabricating semiconductor devices and more particularly, to a method for realizing ultrafine processings as having high selectivity to material and high anisotropy in a surface treating procedure such as etching or deposition and also to an apparatus for .carrying out the method. The term "ultrafine processings" used herein is intended to mean a surface treatment at the level of an atomic layer.
As semiconductor devices are now progressing in fineness, there is a demand for an etching procedure which ensures a higher aspect ratio (i.e. a ratio between an etching depth and an aperture size), with a higher selectivity to material.
In dry etching, a recent trend is to use ions or radicals of an etching gas which are produced in a plasma by application of a high frequency in the order of magnitude of MHz. The chemical species, such as ions, radicals, dissociated molecules and the like, generated in the procedure are various in kind and energy. For instance, CF.sub.4 gas is dissociated in the following way ##STR1##
In the above sequence, F* represents a radical and CF.sub.3 (+) represents a monovalent positive ion of CF.sub.3. Thus, such a simple molecule as CF.sub.4 will be changed into various chemical species in plasma. Additionally, in the prior art procedure, the internal energy of the respective chemical species is not controlled at all. However, although not satisfactory in anisotropy, the above procedure is advantageous in making use of the active species in large quantities, permitting etching to proceed at a high throughput.
This is true of the deposition procedure. More particularly, although thin films can be formed at a high throughput, the chemical species used are not selected and the internal energy is not controlled in this case.
In plasma etching, attempts have been hitherto made to effect a good quality of etching by use of chemical species effective in the etching while eliminating other chemical species unnecessary for the etching. For one instance, there may be mentioned a method set out in Japanese Patent Laid-open No. 62-248226. In this method, various ion species from a discharge plasma are subjected to mass separation by means of a quadrupolar electromagnet, thereby selectively taking out a specific type of ionic species having a desired energy. Subsequently, the resultant ion species are appropriately controlled in velocity by use of deceleration electrodes, following by processing a sample surface therewith. For the selection of ionic species, it is typical to use a quadrupolar mass separator using a high frequency and a Wien filter-type mass separator wherein a magnetic field and an electric field are intersected at right angles. For mass spectrometry, there is known a method which makes use of a quadrupolar ion trap as set forth in Japanese Patent Laid-open No. 59-134546. A light-assisted internal energy control method has been proposed, for example, in Japanese Patent Laid-open No. 61-231716. This method comprises irradiating light on a sample surface which is undergoing the etching reaction, thereby contemplating a good quality of etching.
In the known drying etching processes, there is used a plasma which is generated by application of a high energy. The resultant ions contain different types therein. Accordingly, the internal energies of rotation, vibration and electron of the ions and translational energies thereof vary widely. Among these ions, there are invariably included ions which are not necessary for etching or which are in an energy state where the etching reaction is unlikely to occur. Such ions undesirably act to etch a mask or an underlying layer, making it difficult to obtain high selectivity to material. In addition, since there are used ions which are accelerated and have a high translational energy in order to enhance the anisotropy of an etching shape, the selectivity to material will further lower. This will cause a variety of defective damages of the substrate. Moreover, the etching reaction becomes more complicated by the influence of various radicals and electrons generated along with the ions and are thus more unlikely to control.